The present invention relates to a gas turbine engine, and more particularly to a turbofan engine having a variable area fan nozzle (VAFN) with an acoustic system to attenuate leading edge noise and reduce the total effective perceived noise level (EPNL).
Gas turbine engines which have an engine cycle modulated with a variable area fan nozzle (VAFN) provide a smaller fan exit nozzle diameter during cruise conditions and a larger fan exit nozzle diameter during take-off and landing conditions.
The VAFN may generate significant noise as upstream turbulence interacts with the leading edge of the VAFN. The upstream turbulence may result from turbulent boundary layers which expand from the upstream fixed nacelle wall, turbulence which evolves from the upstream fan exit guide vane (FEGV) wakes or endwall effects, and flow separation that may occur from the contour of the upstream nacelle wall. The physical mechanism for leading edge VAFN noise, which exhibits acoustic dipole behavior, is fundamentally different from traditional jet exhaust mixing noise, which exhibits acoustic quadrupole behavior. Additionally, this excess noise is not significantly reduced in forward flight as typical jet exhaust mixing noise. Thus, the leading edge source is of significant importance in its contribution toward the effective perceived noise level (EPNL).